net: ethernet: stmmac: fix altr_tse_pcs function when using a fixed-link

In the Linux kernel, the following vulnerability has been resolved:

net: ethernet: stmmac: fix altr_tse_pcs function when using a fixed-link

When using a fixed-link, the altr_tse_pcs driver crashes
due to null-pointer dereference as no phy_device is provided to
tse_pcs_fix_mac_speed function. Fix this by adding a check for
phy_dev before calling the tse_pcs_fix_mac_speed() function.

Also clean up the tse_pcs_fix_mac_speed function a bit. There is
no need to check for splitter_base and sgmii_adapter_base
because the driver will fail if these 2 variables are not
derived from the device tree.